1. Field of the Invention
This invention relates to the field of aircraft flight control systems. More particularly, it pertains to a system for trimming control surfaces during transitions between conditions when the craft is airborne and when its landing gear are firmly in ground contact.
2. Prior Art
The main rotor of a helicopter rotatably supports blades having an airfoil shape, which produce aerodynamic lift or thrust as the blades pass through the air. A pitch link attached to each blade changes the angle of attack by applying control force to the blade and rotating it-about its pitch axis, thereby affecting the magnitude of lift produced by the rotor. The opposite end of each pitch link is connected to a rotating swashplate, which is connected to a stationary, nonrotating ring located below the rotating ring by bearings, which allow relative rotation of the rings and hold them at the same angle and relative axial position along the rotor shaft. The stationary ring can be raised and lowered along the axis of the rotor shaft, or tilted with respect to that axis by action of control servos or actuators, a longitudinal servo and multiple lateral servos.
To change the angular position of rotor lift, the pitch of each blade is changed individually, i.e., cyclic pitch is applied by causing the longitudinal servo to tilt the rings and main rotor about the rotor shaft. To change the magnitude of rotor lift, the pitch of all blades is changed concurrently by raising the rings along the rotor shaft by the same amount, i.e., collective pitch is applied by causing the lateral servos to raise the rings relative to the main rotor.
To prevent a single rotor helicopter from rotating continually about its rotor axis, a tail rotor is used to produce a thrust force directed laterally that compensates for main rotor torque. This stabilizes the yaw heading and attitude of the aircraft against wind gusts and changes in main rotor torque. By overcompensating and undercompensating for these transients, the pilot changes the angular position of the aircraft about the yaw axis.
The magnitude of the tail rotor thrust varies with changes in pitch or angle of attack of the tail rotor blades resulting from raising and lowering a rotating swashplate connected by pitch links to the blades. The position of the swashplate is changed while maintaining its angular position constant so that tail rotor blade pitch changes collectively. Conventionally, the tail rotor thrust is controlled by pilot manipulation of control pedals connected by cables, bellcranks and push-pull rods to the tail rotor controls.
Fixed wing and rotary wing aircraft flight control systems include mechanical linkages between the cockpit controls (such as a center stick, pedals or control wheel) and the control surfaces (such an aileron, elevator or rudder for fixed wing aircraft, or longitudinal cyclic pitch, lateral cyclic pitch, main rotor collective pitch and tail rotor collective pitch for a helicopter). More recently, fly-by-wire aircraft controls systems, which usually employ complex stability augmentation systems to significantly reduce pilot workload, have become increasingly prominent. In fly-by-wire systems the mechanical links between the cockpit control and control surfaces may be replaced with electronic systems including sensors, logic and actuators.
Although fly-by-wire flight control systems offer significant improvement over conventional control systems, they present design challenges. Control law architecture and level stability augmentation, which are optimized for in-flight modes of operation, may not be appropriate for air-to-ground and ground-to-air transitions. Therefore, a complete and comprehensive design solution should include a flight control law that specifically accommodates these transitions.
Conventional helicopter flight control systems employ a displacement cockpit controller, such as a center stick, whose position provides an indication to the pilot of the angular disposition or attitude of the rotor. With a displacement controller, there is full correlation between the position of the controller and control surface command. A unique-trim controller does not produce a fixed amount of output for a given pilot input; instead the controller integrates the input over time and produces an output that is a combination of controller displacement and the length of the period of its displacement. Therefore, the direct correlation between controller position and control surface command that is present with a displacement controller is absent with a unique-trim controller.
Rotational constraint of the aircraft about reference coordinate axes due to ground contact impairs the pilot's awareness of control surface command, particularly where a unique-trim controller is employed. A comprehensive solution should resolve this difficulty also.